1. Field of the Invention
This invention relates to a television signal converting apparatus for converting a high definition television signal into a conventional television signal and more particularly, to a television signal converting apparatus having a non-linear level correction means capable of precisely reproducing the signal level of a luminance signal of a high definition television signal which is transmitted non-linearly.
2. Description of the prior Art
A high definition television signal has a frequency band of at least 20 MHz, so that when it is transmitted by a direct broadcasting satellite or the like, it must be subjected to bandwidth compression by an appropriate method. As an effective technique to largely compress the frequency bandwidth of a high definition television signal, the MUSE (Multiple Sub-Nyquist Sampling Encoding) method has been proposed. (See, for example, Y. Ninomiya, et al; "An HDTV Broadcasting System Utilizing a Bandwidth Compression Technique - MUSE", IEEE Trans. Vol. BC-33, No. 4 p. 130 (1987).)
The MUSE technique has adopted as a transmission system the so-called quasi constant luminance transmission system. In this system, an input video signal is encoded and decoded substantially linearly based on the MUSE method and transmitted non-linearly. By performing only the transmission of a video signal non-linearly, noise effects generated in transmission lines have been reduced. The quasi constant luminance transmission system is disclosed, for example, in Japanese Laid-Open patent application No. 63-136790, entitled "Component Video Signal Transmission Method".
To receive a bandwidth-compressed MUSE signal, an appropriate receiver (MUSE decoder) must be used. However, such a receiver is expensive, so that it is estimated that it will take a considerably long period of time to make it popular for home use. As a result, in order to enjoy accepting the high definition television broadcasting service using a commercially available conventional television receiver, there is a signal converting apparatus available, which converts a MUSE signal into a conventional television signal. As a system to be used for this purpose, a circuit structure shown in FIG. 6 can be pointed out as an example. In FIG. 6, a signal converting circuit 2 receives from a MUSE signal input terminal 4 a MUSE signal which has been transmitted non-linearly to change the number of scanning lines thereof for generating a luminance signal and chrominance signal of the conventional television system and sends them to an inverse matrix circuit 3. The inverse matrix circuit 3 converts the luminance Y signal and the chrominance signals C1 and C2 outputted from the signal converting circuit 2 into red, green and blue (rgb) signals and sends them to a picture display device 5.
If a CRT is used as the picture display device 5, due to the difference between the non-linear characteristic that the CRT possesses and the non-linear level expansion characteristic carried out for non-linear transmission on the transmission side, a linear relationship cannot be established between an input signal level of a camera on the transmission side and a display signal level on the CRT display. As a result, the level of a luminance signal cannot be precisely reproduced on the receiving side.
In addition, as a signal converting apparatus for precisely reproducing the level of a luminance signal, a circuit structure as shown in FIG. 7 can be pointed out as an example. In FIG. 7, a transmission inverse gamma correction circuit 21 possesses such an input-output characteristic that the gradient of its curve is increased with an increase in the input signal level as shown in FIG. 8. Such an input-output characteristic is complementary to the non-linear level expansion characteristic to be carried out for the MUSE signal on the transmission side in order to transmit it non-linearly. By possessing the input-output characteristic as shown in FIG. 8, the transmission inverse gamma correction circuit 21 executes a transmission inverse gamma correction process for a MUSE signal supplied from a MUSE signal input terminal 4 for compressing the signal level thereof, thereby making the MUSE signal, which is transmitted under a state that the signal level is being expanded, into a linear signal. Then, the linear signal is sent to a signal converting circuit 2. The signal converting circuit 2 carries out the scanning line conversion of a video signal having 1125 scanning lines per frame sent from the transmission inverse gamma correction circuit 21 to generate a luminance signal Y and chrominance signals C1 and 2c of the conventional television system and sends them to a matrix circuit 3. The inverse matrix circuit 3 converts the chrominance signals C1 and C2 and luminance signal Y sent from the signal converting circuit 2 into rgb signals and sends them to a CRT gamma correction circuit 23. The gamma correction circuit 23 has an input-output characteristic as shown in FIG. 9. The gamma correction circuit 23 executes the CRT gamma correction process to expand the signal levels of the rgb signals outputted from the inverse matrix circuit 3 in response to the input-output characteristic as shown in FIG. 9 in order to correct for the non-linear characteristic that the CRT possesses, thereby sending the thus corrected RGB signals of the conventional television system to a picture display device 5.
The signal converting apparatus shown in FIG. 7 can reproduce precisely the signal level of a luminance signal. For this, however, it is necessary to have at least two kinds of non-linear processing circuits such as the transmission inverse gamma correction circuit 21 and the CRT gamma correction circuit 23. As a result, the circuit scale becomes large, which means that a problem arises in that the signal converting apparatus itself contributes to an increase in cost.